1. Field of the Invention
The present invention relates to a heat-assisted magnetic recording method, which performs magnetic recording by irradiating a light to a magnetic recording medium to reduce an anisotropy field of the magnetic recording medium, and a thin-film magnetic head, which writes data using the heat-assisted magnetic recording method.
2. Description of the Related Art
With increasing recording density of magnetic disk drive apparatuses, it is required to improve the performance of thin-film magnetic heads. Composite-type thin-film magnetic heads, which have a stacked structure of a magnetoresistive (MR) element for reading data signals and an electromagnetic transducer for writing data signals, are widely used as such thin-film magnetic heads.
Generally, magnetic recording media are magnetically discontinuous, in which magnetic microparticles are gathered together, and each of magnetic microparticles has a single magnetic-domain structure. Here one recording bit consists of a plurality of magnetic microparticles. Therefore, for improving its recording density, irregularity in boundaries of recording bits should be reduced by decreasing the size or volume of magnetic microparticles. However, thermal stability of the magnetization of recording bits is degraded by decreasing the size of magnetic microparticles.
As a measure against the thermal stability problem, it may be possible to increase a magnetic anisotropy energy KU of magnetic microparticles. However, increment of the energy KU causes increment of a coercive force of magnetic recording media. Whereas, the intensity of a write magnetic field of thin-film magnetic heads is limited by saturation magnetic flux density of soft-magnetic pole material, which forms a magnetic core of heads. Therefore, heads cannot write data to magnetic recording media when the coercive force of media exceeds the maximum limit of the write magnetic field.
Currently, as a method for solving the thermal stability problem, a heat-assisted magnetic recording technique is proposed, in which a magnetic head writes data to a magnetic recording medium formed of a material with large magnetic anisotropy energy KU by supplying a heat to the medium to reduce the coercive force of the medium just before applying the write magnetic field. The heat-assisted magnetic recording technique has some similarity to a magneto-optic recording technique. However in the heat-assisted magnetic recording technique, the area of applied magnetic field determines spatial resolution of recording bits (that is, magnetic-field-dominant technique), while the area of emitted light determines spatial resolution of recording bits (that is, light-dominant technique) in the magneto-optic recording technique.
As proposed heat-assisted magnetic recording techniques, U.S. Pat. No. 6,768,556 discloses a near-field light probe, which has a strobilus shaped metal diffuser formed on a substrate and a dielectric material film formed around the diffuser, as an emitting unit for irradiating light to the magnetic recording medium. Japanese patent Publication No. 10-162444A discloses a head using a solid immersion lens in a recording and reproducing apparatus. Further, Japanese patent publication No. 2004-158067A discloses a diffuser as a near-field light probe, which is formed in contact with a main magnetic pole of a head for perpendicular magnetic recording in such a way that a irradiated surface of the diffuser is perpendicular to a medium surface. Furthermore, Miyanishi et al. “Near-field Assisted Magnetic Recording” IEEE TRANSACTIONS ON MAGNETICS, Vol. 41, No. 10, p. 2817-2821 (2005) discloses a U-shaped near-field light probe formed on a quartz crystal slider. Further, Japanese patent publication No. 2005-4901A discloses a technique, which can apply an appropriate write magnetic field to a heated area of a magnetic recording medium by manipulating a gradient of the write magnetic field and so on, even though a light emitting unit is provided around an trailing side end surface of a main magnetic pole.
As described above, various forms of heat-assisted magnetic recording techniques are proposed. However, following problems arise to realize heat-assisted magnetic recording using a near-field light generating element such as the near-field light probe described above.
In case the near-field light generating element is provided on the trailing side (opposite side to the substrate) with reference to the main magnetic pole of the head, the irradiating center of the near-field light should be closed to the main magnetic pole enough. For example, in case the trailing side gradient of the write magnetic field profile, which is a intensity distribution of the write magnetic field along the track, is 100 Oe(Oersted)/nm, the maximum of the write magnetic field is 10 kOe, and a write magnetic field more than or equal to 5 kOe is required for writing to the magnetic disk, a distance between the main magnetic pole and the irradiating center of the near-field light need to be adjusted less than or equal to 50 nm. Further, in case the gradient of magnetic field need to be increased, for example, by 500 Oe/nm for high recording density, the distance need to be further decreased.
On the other hand, the near-field light generating element generally generates a near-field light by receiving a light propagated through the waveguide. The waveguides is formed by surrounding a higher refractive index region (core) using a lower refractive index region (clad). To keep functions as the waveguide, a thickness of each region need to be set almost the same as or more than the wavelength of the light to be propagated. In case of using a blue laser, which is normally used for the high density optical recording, a thickness of the clad need to be approximately 400 nm or more, and the efficiency of the light propagation is dramatically degraded using clad thinner than 400 nm. As a result, the near-field light generating element, which is provided at the end surface of the waveguide, cannot be placed close enough to the main magnetic pole.
It is considerable to place the near-field light generating element on the leading side (substrate side) with reference to the main magnetic pole. For example, Japanese patent publication No. 2005-190655A discloses a configuration, in which a light emitting element as a heat source is provided on the leading side of the magnetic pole for writing. In this case, recording bits can be damaged, because magnetization transition regions of recording bits are disturbed by receiving a higher write magnetic field after writing, in addition to the difficulty of shortening the distance between the near-field light generating element and the main magnetic pole.
On the contrary, Miyanishi et al. “Near-field Assisted Magnetic Recording” IEEE TRANSACTIONS ON MAGNETICS, Vol. 41, No. 10, p. 2817-2821 (2005) proposes a structure, in which a near-field light generating element and a write magnetic field generating element are placed at the same point. However, this structure does not include a main magnetic pole, and an applicable write magnetic field is limited. Moreover, it is difficult to realize the near-field light generating element with enough generating efficiency so far, in addition to problems listed above. Especially, the generating efficiency is not enough to supply the appropriate heat to the magnetic recording medium, which rotates high speed, for example approximately 7200 rpm.
From above considerations, the heat-assisted magnetic recording without using the near-field light generating element is expected. However, it is not possible to realize the good heat-assisted magnetic recording by just using a light supplying unit, which can supply a light with a big spot diameter. For example, in case the light supplying unit, which can supply the light with a big spot diameter, is placed on the trailing side with reference to the main magnetic pole, recording bits may be damaged, because magnetization transition regions of recording bits are disturbed by being exposed to higher temperature after writing. On the other hand, in case the light supplying unit is placed on the leading side with reference to the main magnetic pole, magnetization transition regions of recording bits are eventually decided under the condition that gradients of both the magnetic field and the temperature are small, and therefore it is very difficult to achieve a high line recording density.